There are known methods for thermally controlling characteristics of optical devices by applying heat to the optical devices. For example, the resonant wavelength of a ring resonator can be controlled by heating the ring resonator. Also, the free spectral range (FSR) of a wavelength filter can be thermally controlled by heating the wavelength filter.
Further, there is a known technology where light output from a light-emitting device such as a laser diode disposed on a substrate is received by a light-receiving device via an optical waveguide and optical devices such as a ring oscillator and a wavelength filter whose characteristics are controlled by heating them with a heater (see, for example, US Patent Application Publication No. 2008/0193145 and US Patent Application Publication No. 2013/0094527).
However, when a ring oscillator and a wavelength filter are disposed on a single substrate and characteristics of both of the ring oscillator and the wavelength filter are thermally controlled, heat applied to the ring oscillator and heat applied to the wavelength filter interfere with each other, and desired characteristics may not be obtained.